This invention relates to a process for remediation of environmental contamination in subsurface soils. More particularly, this invention relates to remediation processes involving emplacement of solid-phase or other nonaqueous-phase treatment agents with soil fracturing technology. Emplacement of chitin as an electron donor for microorganisms that carry out reductive dechlorination of chlorinated solvent source areas or plumes in low permeability media is illustrative of the invention.
Chlorinated solvents are the most common class of ground water contaminants at hazardous waste sites in the U.S. In a list of the top 25 most frequently detected contaminants at such sites, the Agency for Toxic Substances and Disease Registry (ATSDR) found that 10 of the top 20, including two of the top three, were chlorinated solvents or their degradation products. National Research Council, Alternatives for Ground Water Cleanup (National Academy Press, Washington, D.C. 1994). In fact, the same survey found that the most common contaminant, trichloroethylene (TCE), is present in more than 40% of National Priority List sites. Worse yet, remediation of ground water contaminated by these compounds often presents unique obstacles related to their inherent characteristics, including hydrophobicity and high density.
Overcoming these obstacles often demands innovation and an interdisciplinary approach that integrates hydrology, geology, chemistry, microbiology, and economics. In particular, an innovative approach has been conceived, and is described herein, to harness recent advances in the understanding of biodegradation processes involving chlorinated solvents for remediating residual source areas, or for cutting off dissolved plumes, by emplacing solid-phase or other nonaqueous-phase treatment agents into a variety of soil types throughout much larger volumes of the subsurface than has been possible using conventional methods. One embodiment of this innovation involves delivering chitin, as an electron donor, into induced fractures in low permeability soils to create and maintain nutrient-rich anaerobic conditions that will promote the long-term bioremediation of a chlorinated solvent or other dense nonaqueous phase liquid (DNAPL) source.
Chitin is the structural component of the shells of crustaceans. Chitin is also present in many other living organisms, such as insects and mushrooms. Because it is present in so many different types of living organisms, chitin is the most plentiful natural polymer next to cellulose.
Chitin is a condensation polymer of N-acetyl-D-glucosamine where minority of the acetyl groups has been lost. Chitosan refers to a deacetylation product obtained from chitin where most of the acetyl groups have been removed. Experimentally, chitosan can be distinguished from chitin because of its solubility in dilute acetic or formic acid. Also, chitin contains less than 7% nitrogen, while chitosan contains 7% or more nitrogen. The amino groups of chitin and chitosan are exceptionally stable in 50% sodium hydroxide, even at high temperature. Glucosamine occurs as an essential part of the polymer structure. Elemental analysis of chitin samples reveal that they bind water tenaciously. This is consistent with the general picture of chitin structure as a chain of N-acetylglucosamine punctuated by free glucosamine units with considerable amounts of trapped water as part of the molecule.
Chitin and chitosan are both biodegradable and non-toxic, and they have binding properties such that they function as excellent flocculants for clarifying liquids, help heal wounds, can be fabricated into strong permeable films, and function as drug-delivery gels for topical application of a variety of medicaments. It has also been determined that chitin may be used as an electron donor in bioremediation of contaminants in the environment.
In view of the foregoing, it will be appreciated that providing a process combining fracturing technology with use of solid-phase or other nonaqueous-phase treatment agents for remediation of environmental contamination in subsurface soils would be a significant advancement in the art.